1. Field of the Invention
The invention relates more particularly to electric oscillators of the type comprising a surface wave resonator also known as "SAW".
These oscillators have at least one SAW resonator with two pairs of interdigitated electrodes mounted on a substrate, very often of quartz, between which surface acoustic waves are propagated and an active circuit connected to said electrodes.
SAW oscillators have numerous ultra high frequency applications and have the advantage of high spectral purity. For example, in the frequency range between 100 and 1000 MHz, they give a phase noise gain of more than 20 dB with respect to volume wave quartz oscillators.
The problem solved by the invention is that of the frequency setting of an oscillator and more particularly of a SAW oscillator.
The frequency of a SAW depends on a number of dimensional factors (in particular the distance between the adjacent fingers of the interdigitated electrodes and on the elastic constants of the substrate.
The initial frequency setting of the SAW at a reference temperature depends on the dimensional accuracy of a mask ruled with a large number of lines, all the more since the quality factor (overvoltage due to resonance) of the SAW is high. Moreover, the subsidiary electric setting which can be provided by the circuit of the oscillator is substantially less than the frequency dispersion of the SAW and provides a correction all the poorer the higher the quality factor.
The result is finally that a SAW resonator is, by construction, imprecise in frequency and all the less adjustable by electric control as its quality factor is high, apart from resorting to sorting out which results in an unacceptable wastage rate.
By way of example, a 200 MHz SAW resonator with an overvoltage of 30,000 is, by construction, imprecise to .+-.20 Kz, whereas its electric resetting is only .+-.10 KHz. To that must be added a longterm drift of .+-.5 KHz per year and a temperature coefficient, because the temperature affects not only the dimensional factors but also the elastic constants of the substrate.
To avoid having to place the SAW in a thermostatic enclosure, it has already been proposed to minimize its proper temperature coefficient, either by using particular crystallographic cut of the substrate, (see for example U.S. Pat. Nos. 4,400,640, 4,602,182 and 4,609,843), or by a particular construction of the substrate (U.S. Pat. No. 4,622,855), or by associating several SAWs to provide mutual temperature compensation (FRA No. 2 392 538). Different electric circuits have also been proposed servo-controlled to the temperature of the substrate and adapted for controlling the frequency thereof within the possible resetting range (U.S. Pat. Nos. 4,491,931, 4,489,289).